Community Stability and the Strategy of Biological Control

Abstract

Canad. Ent. 97: 887-895 (1965) Community organization is defined as the mean number of trophic links connecting species of different trophic levels in a community. For the special purposes of this paper, competition is assumed to occur whenever two species are known to eat the same food species. Community stability is defined as the reciprocal of the mean, for all species, of the standard error of logarithms of annual collection sizes. It is thus a measure of stability over time of the species populations in a community. Several authors writing about the relationship between community organization and community stability have insisted that the stability of a complex ecological system increases with the number of avenues by which energy can flow through it. This theory does not seem consistent with the observation that some insect pest species of notorious ~nstabiiity are attacked by a great number of entomophagous species. In this paper we seek additional evidence bearing on the relationship behveen community organization and stability, using computer analysis of data collected by the Canadian Forest Insect Survey on forest Macrolepidoptera and their food plants. After cnnsidering our results, and those of other workers, 7,ve postulate the fouc~wing hypothesis. Stability at any herbivore or carnivore nopllic level increases with the number of competitor species at that level, decreases with the number of competitor species that feed upon it, and decreases with the proportion of the environment containing useful food. If this hypother,is is valid. too much competition in the entomophagous trophic level will not all(>a the species in that level to be instable enough to control an unstable pest. Thercfore, the best type of biological control agent is one that has no direct comp-titor species. :Use, the most unstabk biological control agents, and hence those capable of controlling an unstable pest, will be polyphagous.

Disturbance, Competition, and Herbivory Effects on Ragwort Senecio Jacobaea Populations

“…Some have suggested that interspecific interference may cause the combined effect of multiple enemy species to be less than the effect of the single most efficient enemy species (Turnbull and Chant 1961, Watt 1965, Kakehashi et al 1984. -A second controversial issue in biological control is whether it is a better strategy to introduce a single "best" enemy species or whether several natural enemy species should be introduced for greatest effect.…”

Potential for intraguild predation and competition among predatory Coccinellidae and Chrysopidae

“…Predation and parasitism are the cornerstones of biological control (DeBach, 1964), but there is no consensus on the role of competition in biological control. Some researchers contend that species interactions through (within-guild) interspecific competition may reduce pest suppression (Ehler, 1985;Force, 1974;Watt, 1965); whereas others have demonstrated species interactions that led to better pest density suppression, as a result of complementary actions (Huffaker & Kennett, 1966;Huffaker, 1986). The importance of mutualism in biological control is observed primarily when predation and/or parasitism of homopterans are reduced when homopterans are tended by ants (Hölldobler & Wilson, 1990).…”

Biodiversity and habitat modification in pest management

“…Fertiliser perturbation of a field which had been abandoned for 16 years was found to cause a surge in herbivorous pests. A survey in Canadian forests has shown that simple parasitoid complexes may lead to greater stability of Lepidoptera than larger complexes which share more intense competition (Watt, 1965) and, under very controlled laboratory conditions, an aquatic protozoan (Paramecium) was kept more stable if the number of its competitor species for food or its predator species was reduced (HairstOn et al, 1968). The stabilityof species abundance and composition from year to year of three fields in Michigan with the same history of abandonment was inversely related to diversity (Murdoch et al, 1972).…”